1. Field of the Invention
The present invention relates to a voltage-dependent non-linear resistor member, a method for producing the same and an arrester equipped with the member. More specifically, the present invention relates to a voltage-dependent non-linear resistor member and a method for producing the same, wherein the resistor member comprises a sintered material, the principal ingredient of which is zinc oxide, and is practically available for the material of an arrester, a surge absorber, and others.
2. Description of the Related Arts
Conventionally, a voltage-dependent non-linear resistor member which principally consists of zinc oxide and is used as an arrester and the like comprises a sintered material produced by means of granulation, compacting, and burning from a mixed composition of zinc oxide which is the principal ingredient, bismuth oxide which is considered as essential to expression of voltage-dependent non-linear resistance, and other additives which are effective for improvement of electric properties. Further, the sintered material is provided with a high-resistance side layer and electrodes comprising metal aluminum and/or the like to make up the resistor member (see; FIG. 6).
FIG. 7 is a schematic drawing illustrating a micro-structure of a part of crystal structure of an ordinary voltage-dependent non-linear resistor member. In the figure, the numeral 1 indicates spinel grains mainly constituted by zinc and antimony, 2 indicates zinc oxide grains, 3 indicates zinc silicate, Zn.sub.2 SiO.sub.4, 4 indicates bismuth oxide, and 6 indicates twinning boundaries in zinc oxide crystal grains. Specifically, the spinel grain principally consisting of zinc and antimony may take either of two existing states in the structure, namely, some spinel grains exist surrounded by zinc oxide grains 2, while others exist near triple points (multiple points) of zinc oxide grains. Further, some of bismuth oxide 4 exist at the boundaries of zinc oxide grains 2 as well as at the multiple points.
An experiment using point electrodes has revealed that a grain itself which principally consists of zinc oxide functions as a mere resistive substance while exhibiting voltage-dependent non-linearity at the boundary portion between the zinc oxide grain 2 and another zinc oxide grain 2 (G. D. Mahan, L. M. Levinson & H. R. Philipp, "Theory of conduction in ZnO varistors", J. Appl. Phys. 50 4!, 2799 (1979); hereinafter referred to as Reference 1). Additionally, it is also experimentally confirmed that the number of the boundary portion between zinc oxide grain-zinc oxide grain (grain boundary) determines the varistor voltage (T. K. Gupta, "Application of Zinc Oxide Varistors", J. Am. Ceram. Soc., 73 7!, 1817-1840; hereinafter referred to as Reference 2; or others).
FIG. 8 is a diagram showing a voltage-current characteristic (non-linearity characteristic) of an ordinary voltage-dependent non-linear resistor member having the above-described micro-structure.
Zinc oxide voltage-dependent non-linear resistor members having excellent protective performance possess a small V.sub.H /V.sub.L ratio (limit voltage ratio, or flatness ratio), wherein V.sub.H and V.sub.L are values of voltages at a large-current region and a small-current region in FIG. 8, respectively. When improvement in limit voltage ratio is discussed, the limit voltage ratios in the large-current region and the small-current region should be each individually discussed since the factor which determines the limit voltage ratio in one of said regions is different from the factor which determines the limit voltage in the other region. Therefore, hereinafter, the limit voltage ratio V.sub.H /V.sub.L is separately discussed using the voltage, V.sub.S at S of FIG. 8 in each view of the flatness ratio in the large-current region V.sub.H /V.sub.S or the flatness ratio in the small-current region V.sub.L /V.sub.S, respectively.
As to the flatness ratio in a large-current region V.sub.H /V.sub.S, V.sub.H is believed to be determined by internal resistivity of a zinc oxide crystal grains (References 1 and 2). V.sub.H decreases in accordance with decrease in the internal resistivity of a zinc oxide crystal grain, and therefore, V.sub.H /V.sub.S would be also smaller. On the other hand, the flatness ratio in a small-current region V.sub.S /V.sub.L is believe to be determined by a Schottky barrier which is considered to be formed at the grain boundary between zinc oxide crystals (References 1 and 2). As the apparent resistivity at the grain boundary between zinc oxide crystals becomes large, V.sub.S /V.sub.L becomes smaller. Accordingly, it is suggested that internal resistivity in a zinc oxide grain should be decreased and apparent resistivity at the grain boundary between zinc oxide crystals should be enhanced to improve the discharge voltage, V.sub.H /V.sub.L.
The V.sub.S indicated in FIG. 8 is the non-linear threshold voltage in voltage-dependent non-linear resistor members. The value of V.sub.S is determined corresponding to the transmission system to which an arrestor is applied. In many cases, V.sub.3mA is used as a typical value for V.sub.S, wherein V.sub.3mA is an inter-electrode voltage between both ends of a device when 3 mA of electric current is applied to the device. Taking account of the size of the device, the current value of 3 mA equals approximately 50 .mu.A/cm.sup.2 of current density. The V.sub.S value of a zinc oxide device is in proportion to the thickness of the device.
In apparatus used with a high system voltage, for example, an arrestor used for electrical power transmission at UHV 1 MV, the number of series-laminated devices increases when devices which have a uniform shape and a V.sub.S value similar to that of conventional devices are laminated. As a result, the size of the arrestor becomes large, and the mode for series connection will be complicated, and therefore, many problems arise in relation to electrical matters, thermal matters, and mechanical design. Accordingly, these problems can be solved if a device which has a large V.sub.S value per unit length (for example, V.sub.3mA /mm: varistor voltage) is available, since the distributed voltage per device becomes high and the number of series-laminated devices can be reduced. Here, V.sub.S value per unit length is calculated by dividing the V.sub.S value by the thickness value of the device.
A prior investigation has revealed that the factor which controls V.sub.S value is the sizes of zinc oxide grains 2 in the crystal structure of a device shown in FIG. 7 (Reference 2). The region around 3 mA is the non-linear region in the voltage-current characteristic shown in FIG. 8, and the below-described equation I holds true experimentally: EQU V.sub.3mA /mm=k/D I,
wherein k is a constant and D is a mean grain size of zinc oxide. Accordingly, 1/D equals the number of grain boundaries between zinc oxide grains per unit length, Ng. The above equation I can be thus expressed as the below-described equation II. EQU V.sub.3mA /mm=k'D II
It is obvious that the constant k' represents the varistor voltage per grain boundary of the zinc oxide device (Reference 2).
In summary, at least two requirements as follows can be listed to accomplish excellent protective properties:
i) as to the electrical properties of the varistor, limit voltage ratio, V.sub.H /V.sub.L is small; and, PA1 ii) as to the electrical properties required of a voltage-dependent non-linear resistor member for a practicable arrestor having a compact size, the varistor voltage is made large. Additionally, when the shape of the device is the same as that of conventional one, it is naturally required to have a large value of energy bearing capacity in proportion as the varistor voltage of the device increases. Since the factor which determines the protective properties of arrestors is relative to the above-described i), it is particularly required to reduce the limit voltage ratio by improving the composition of the voltage-dependent non-linear resistor member and/or process for producing the same. Further, since the factors which determine the features of the arrestor such as size are relative to the above-described ii), it is particularly required to render the varistor voltage large.
The present invention has been achieved to solve the above-described problems. Therefore, an object of the present invention is to provide a voltage-dependent non-linear resistor member, a method for producing the same, and an arrester equipped with the same wherein the resistor member has high varistor voltage and small limit voltage ratios, namely, excellent flatness ratios throughout the large- and small-current regions. Further, another object of the present invention is to provide a voltage-dependent non-linear resistor member having a large varistor voltage and a method for producing the same.